Gryphaea – Continued
My suggestion in last years journal that Gryphaea might have lived in a fairly molluscan fashion with its domed shell uppermost, was roundly deprecated by several members of the Society, but others, together with some at Bristol, thought that the suggestion might deserve a little further investigation during the summer.
As a result, one afternoon last August a small party which included a very enthusiastic young lady of nearly eleven, who will, I’m sure, one day be a scientist, set out for Hock Cliff with the intention of gathering 100 specimens for examination at home, although tide, rain and mud ultimately defeated us and only 78 were collected. These were a very mixed lot, deliberately unselected, all dirty, some broken, but with sufficient clues, 12 with umboes damaged or broken off, 28 all but completely complete to be identified as Gryphaea arcuata cleaning in the garden water butt the following day.
Although not many were text-book specimens, they nearly all contributed some information when they were examined. First of all, we had clearly collected two distinct species in nearly equal numbers, 35 Gryphaea arcuata Lamarck and 27 a lighter species, the remainder being inconclusive, although this might be slightly misleading as there were also seven teeth of the less robust type, Gryphaea arcuata Lamarck was clearly identifiable, comparing very well with the drawings on Plate 21 of ‘British Mesozoic Fossils’ (4th Edition) whereas the other had a thinner shell, umbo turned to a greater angle to the medial line, and a length to width ratio much closer to Gryphaea sublobata Deshayes (Plate 14 of the same reference), although it is unlikely to be so identical owing to the difference between the Lower Lias and the Bajocian where it was once found. It is a pity that these differences were not noticed at an earlier date as it would have been of interest to report whether they lived in separate zones or whether they were exactly contemporaneous. Surely they could not have been mixed at that particular site by the action of present day running water, as the nearest Inferior Oolite is many miles away, and neither species show any sign of tumbling down a stream bed.
It was surprising to note how many of the fossils were either complete, or clearly broken only recently by present day processes of exhumation by weather and tide, the impression being that they must have been buried very quickly after death, before they could be broken either by scavengers or by being tumbled about by currents in the water. How different was the process shown by Dr. Hamilton in his recent lecture, which resulted in a pavement made almost entirely of broken shells. The implication is that these creatures lived on a soft bottom into which they sank very easily when they died, but this raises the question of how they remained above it when they were alive, unless they were capable of taking some action to avoid premature entombment. What better action can be envisaged than to press downwards with the broad operculum they all possessed to raise the main shell, followed by the use of the umbo to hook themselves out.
When shellfish are observed living in present day environments, it is clear that they are prone to gather barnacles or other passengers including fronds of algae on their exposed shells, and where currents are not particularly strong some shellfish can become so encrusted that they are almost invisible on the bottom unless they move. Weed and barnacle accumulation is not exclusive to creatures that live permanently submerged, as those in the intertidal zone can also be affected depending on the violence of the tidal regime, although creatures in such environments usually have some means of attachment to solid surfaces for their protection, such as a recognisable pedicle, a capacity to cement themselves down, or to hold on tightly by suction, but after death these mechanisms give little protection against breakage before the shells reach the fossil record. Other creatures survive by burrowing into sand or mud, which might protect their shells after death, but also prevents the acquisition of algae or barnacles during life, any such marks on isolated specimens of their fossil shells usually denoting a period of post-mortem exposure.
Barnacles, and the scars or specks of calcareous cement where algae were secured, fossilize very readily with the shells that were their hosts, remaining very useful clues to their mode of existence when they were alive, even for shellfish long extinct, provided they are found in unambiguous numbers. In this respect the Gryphaea were very interesting:
a. Only three were found to have barnacle-like growths, one of which was most certainly post-mortem, as it bridged between the operculum and the lip of the main shell.
b. Eight, including all three with barnacle-like growths, had evidence of algae on the operculum.
c. Twenty-eight, including four from ‘b’ had clear evidence of algae on the domed part of the shell.
d. A further ten had marks on the domed part of the shell that might have been algae, but were considered too indistinct for various reasons.
e. Only three had clear marks of weed attachment on the umbo, one being from ‘a’ above.
f. The remainder had no marks of any significance.
Whilst acknowledging that this might have been a statistically improbable sample, it is difficult to reconcile these observations with the concept of a sedentary creature that passed its quiet life umbo and operculum upwards, on a fairly solid floor that presented no danger of engulfment. It is just imaginable that it floated out almost neutral buoyancy on the surface of a high-density, semi-liquid mud, with its umbo projecting into the clear water above, but how then can the weed attachments on the domed part of the shell be accommodated for?.
To me, however, it suggests an animal able to resist by its own efforts the continual prospect of early burial in the mud that became the present Lias, aided by its shape which presented a large flat surface downwards. It was clearly prone to occasional weed infestation on its exposed domed shell, although many would escape this because of the muddy conditions. There were probably isolated harder patches on their living floor, on which they might be stranded in any aspect after death for long enough to acquire a posthumous weed or barnacle on other parts of their shells, but such events would be exceptional rather than normal. It would be an uncompromising environment for creatures less well adapted to it, which would account for the absence of predator damage.
This enquiry was carried out to see whether there is any corroborative evidence for the somewhat tentative suggestions contained in my articles last year, and it would appear from the foregoing that there is a case to be answered. It was fun whilst it lasted, despite the depressing weather, and could possibly be of interest to others if next summer is less pluvial than last.